[1] Latitudinal changes in topography, climate, and thrust belt geometry in the central Andes have led to conflicting hypotheses that climate or tectonics exert a first-order control on orogen evolution. The relative roles of climate and tectonics in the evolution of the Andean orogen are difficult to quantify because of a lack of detailed observations for both the long-term deformation and erosion history of the Andean foldthrust belt. We contribute to the resolution of this problem by presenting a sequentially restored, balanced cross section based on new mapping across the northern Bolivia portion of the thrust belt (15-17°S). The timing and magnitude of exhumation across the cross section are determined by synthesizing 10 new and $70 previously published mineral cooling ages. Once balanced and restored, the section was sequentially forward modeled using stratigraphic and cooling age constraints. Results indicate the Eastern Cordillera (EC) records the highest magnitudes of shortening (123 km or 55%). The Interandean zone (IA) has shortened 48 km or 30%. In both the EC and IA individual thrust sheets are tightly folded and have minor offsets of 1-5 km. The Subandes (SA) has multiple levels of detachments allowing for thrust sheets with relatively large offsets (6-17 km). Total shortening in the SA is 66 km or 40%. Total magnitude of shortening for the entire fold-thrust belt in this region is 276 km (40%). New apatite and zircon fission track cooling ages in conjunction with published ages indicate two phases of rapid exhumation; an earlier phase from $40 to 25 Ma in the EC and one prior to $25 Ma in the IA, followed by distributed exhumation of the entire fold-thrust belt from $15 -0 Ma. Combined exhumation estimates from the balanced cross section and thermochronology suggest $9-11 km of exhumation in the EC, $5-9 km in the IA, and $3-4 km in the SA. Long-term shortening rates are 7 mm/a for the EC and IA and 4-8 mm/a for the SA. The SA shortening rates are based on a $15-0 Ma or 8-0 Ma deformation window. By linking cooling ages to location and magnitude of shortening, we suggest an $10-17 Ma pause or a dramatic deceleration in the rate of deformation and propagation of the fold-thrust belt between 25 and $15 or 8 Ma.

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Quantifying the timing, magnitude, and rates of exhumation and deformation across the central Andes is a prerequisite for understanding the history of plateau rise. We present 23 new apatite and zircon fission track thermochronometer samples to chronicle the exhumation and deformation across the entire (∼500 km) Andean fold‐thrust belt at ∼19.5°S in Bolivia. Exhumation and deformation are constrained with inverse thermal modeling of the thermochronometer data, regional stratigraphy, geothermal gradients, and mass deficits inferred from a balanced section. Results suggest the following: (1) Initial exhumation of the Eastern Cordillera (EC) fore‐thrust and back‐thrust belts began in the late Eocene to early Oligocene (27–36 Ma) and continued in a distributed manner in the late Oligocene to early Miocene (19–25 Ma). Interandean zone (IA) exhumation began 19–22 Ma, followed by a third pulse of exhumation (11–16 Ma) in the EC back‐thrust belt and initial cooling in the westernmost Subandes (SA) 8–20 Ma. Finally, exhumation propagated eastward across the SA during the late Mio‐Pliocene (2–8 Ma). (2) Exhumation magnitudes are spatially variable and range from maximums of <8 km in the EC fore‐thrust belt to average values of ∼4–7 km across the EC, ∼2.5–3 km in the Altiplano, ∼4–6 km in the IA, and ∼3 km in the SA. (3) Exhumation rates range from ∼0.1 to 0.2 mm/a in the EC, from ∼0.1 to 0.6 mm/a in the IA, and from ∼0.1 to 0.4 mm/a to locally 1.4 mm/a or more in the eastern SA. We synthesize similar constraints with sediments throughout Bolivia and characterize plateau development by (1) distributed deformation throughout the Altiplano and EC regions from ∼20 to 40 Ma with minor deformation continuing until ∼10 Ma, (2) contemporaneous cessation of most EC deformation and exhumation of the IA ∼20 Ma implying establishment of the modern plateau width with significant, but unknown crustal thickness and elevation shortly thereafter by ∼15–20 Ma, and (3) dominantly eastward propagation of deformation from the IA since ∼20 Ma with minor out‐of‐sequence deformation in the central to eastern SA.

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[1] Recognition that channel form reflects a river's ability to erode rock and transport material has spawned stream-power models that estimate incision patterns by approximating energy dissipation within a channel. These models frequently assume that channel width scales as a power law with drainage area, partly because drainage area is easily extracted from digital elevation models (DEMs). However, this assumption is often confounded by local variations in rock strength and rock-uplift rate that can cause channel constriction downstream. Here we investigate the morphological response to spatial changes in rock strength and rock-uplift rate of 10 bedrock channels traversing the Mohand range along the northwest Himalayan front. We present a new method to continuously measure and compare channel width, slope, and other hydraulic parameters that integrate satellite imagery and DEM analysis. Our method corrects for an~13% overestimation of average channel gradient from a 90 m resolution DEM that arises from short circuits of fine-scale meanders. We find that channels (1) narrow >1 km upstream from knickpoints formed by an increase in rock strength, (2) adjust laterally more than vertically in response to downstream decreases rock erodibility and uplift rate, and (3) meander where shear stresses are high and channel widths are low. We attribute these results to a high ratio of sediment supply to transport capacity, which enhances lateral erosion relative to vertical incision. Our results suggest that substrate strength and sediment supply substantially influence channel form and that channel width should be explicitly measured when interpreting tectonic signals from bedrock channel morphology.Citation: Allen, G. H., J. B. Barnes, T. M. Pavelsky, and E. Kirby (2013), Lithologic and tectonic controls on bedrock channel form at the northwest Himalayan front,

[1] Constructing an accurate kinematic model for crustal thickening in the central Andes is hampered by the inability of 2-D balanced cross sections to resolve the 3-D displacement field within the poorly studied orocline core. This study presents new structural and thermochronometer data from the orocline axis in the central Andes that constrains the magnitude and timing of deformation through (1) a balanced cross section quantifying shortening perpendicular to strike, (2) field observations that constrain translational displacements parallel to strike, and (3) thermochronology that brackets the age of exhumation and deformation. Mapping results show that north directed, right-lateral motion has been accommodated on at least one major strike-slip fault in the Eastern Cordillera (EC). Shortening perpendicular to fault strike is~179 km (41%) for the EC and inter-Andean zone (IA) and 86 km (39%) for the sub-Andes (SA). Apatite fission track cooling ages indicate rapid EC/IA exhumation from 46 to 18 Ma and 15 to 0 Ma in the SA. Zircon and apatite (U-Th)/He ages show that rapid exhumation began 10-5 Ma in the SA. These results are integrated with existing paleomagnetic and GPS rotation data in map view reconstructions to quantify the magnitude of translational faulting related to oroclinal bending. The reconstructions suggest that at least 85 km of north directed translational displacement has been accommodated within the orocline core. Orogen-parallel displacement and vertical axis rotations have focused crustal thickening at the orocline core and facilitated development of a high-elevation plateau, suggesting a genetic link between large-scale oroclines and plateaus.

[1] Fold topography preserves a potentially accessible record of the structure and evolution of an underlying thrust fault system, provided we understand the factors that shape that topography. Here we examine the morphology and fault geometry of two active folds at the northwest Himalayan front. The Chandigarh and Mohand anticlines show the following patterns: (1) most (∼60%-70%) growth in catchment size and relief (across multiple scales) is accomplished within ∼5 km of the fault tips, (2) range-scale relief is divided unevenly between the fold flanks because of base level contrasts, (3) mean gradients of the uplifting catchments correspond to different flank-averaged rock uplift rates, (4) high hillslope-scale relief coincides with areas of fast rock uplift and stronger lithologies, and (5) existing relief represents only ∼15% of the total rock eroded since faulting began, implying significant erosion. The first-order fold topography is developed quickly and asymmetrically as a result of fault-generated rock uplift (which sets the space available for the fold and the distribution of rock uplift rates) with some modulation by base level (which affects the erosional response of the landscape to the uplift). A linear rate of growth in catchment relief with range half-width correlates with catchment-averaged rock uplift rate, suggesting that this metric may be used to infer variations in fault dip at depth. In these frontal fold settings, high slip rates, weak uplifting rocks, and rapid erosion may combine to quickly limit the topographic growth of emerging folds and disconnect their morphology from the displacement field.